1. Field of the Invention
The present invention relates to a method for treating a noxious gas by a non-thermal plasma, and more particularly to a method for treating a toxic compounds-containing gas by a non-thermal plasma, which removes a possibility of the generation of arc by providing catalysts being electrically less conductive, thereby realizing the generation of a stable non-thermal plasma at a low cost and an enhanced efficiency of the treatment.
2. Description of the Related Art
Almost all of volatile organic compounds (VOCs) exhausted inevitably from most industrial processes are known to do direct harm to the human body and become causal materials generating photochemical smog in the atmosphere. By reason of this, most countries enforce various laws for regulating the VOCs. Also, more and more strengthening restrictions are being imposed on the exhaustion of global-warming materials, such as perfluorocarbons (PFCs) and chlorofluorocarbons (CFCs), in accordance with international agreements, and these materials are expected to be regulated by the total amount of all the materials after the year 2002. Therefore, various methods for treating the toxic compounds are currently used in practice, such as an incineration method, a catalytic method, an adsorption method and a biological method. There, however, is a common recognition that these existing methods do not satisfy requirements for exhausting noxious gases in various industrial processes with respect to the strengthening regulations. For example, each of the incineration method and the catalytic method requires a high-temperature heating source regardless of its type, which makes it difficult to apply these methods to ultra-clean semiconductor processes having technical troubles in providing the high-temperature heating source.
On the other hand, there is an alternative to the incineration method and the catalytic method, a non-thermal plasma method capable of decomposing and oxidizing toxic compounds without using any high-temperature heating source. A plasma is generated by supplying high-voltage DC, AC or pulse power to electrodes opposing each other and thus separating gas molecules having originally electrical neutrality into electrons and cations having polarities. The plasma is divided into two types, namely a non-thermal or low-temperature plasma having tens of thousands degrees of temperature and an ultrahigh-temperature nuclear fusion plasma having tens of millions degrees of temperature. The non-thermal plasma is used more actively in industry than the ultrahigh-temperature nuclear fusion plasma. In the case of the non-thermal plasma, there is no or little increase in process temperature because electrical energy is supplied only to electrons having an ability to use the energy for a chemical reaction by virtue of designs of (1) placing a dielectric barrier between the electrodes, (2) supplying power repeatedly for a time of 1000 nanoseconds so as not to heat the ions or (3) allowing a flow rate of gas molecules passing between the electrodes to be faster. In recent, there is a preference to a method for generating non-thermal plasma by filling electrical dielectrics or strong dielectrics between a pair of electrodes and supplying high-voltage AC power.
A typical example of the present time using the non-thermal plasma under the atmosphere is an ozone-generating device that was developed 150 years ago by Siemens. In the year 1976, Henis disclosed a method for treating a toxic compounds-containing gas by filling dielectrics between metal electrodes opposing each other, supplying high-voltage AC power to generate the non-thermal plasma, and then passing the gas through the non-thermal plasma (See U.S. Pat. No.3,983,021). In addition to the ozone-generating techniques, treating process techniques of nitrogen oxides, VOCs, dioxin, bad-smelling substances, PFCs and CFCs using non-thermal plasma have been developed actively during the past 10 years. Nunez et al. has disclosed that toxic compounds including VOCs can be removed by filling BaTiO3 pellets or beads as the strong dielectrics between metal electrodes of a non-thermal plasma reactor, supplying high-voltage AC power to generate the non-thermal plasma, and then passing toxic compound-containing gas through the dielectrics (See U.S. Pat. No.5,236,672). Birmingham et al. has also disclosed a method for decomposing noxious gas and toxic compounds in a non-thermal plasma reactor by supplying high-voltage AC power. This method is characterized in that dielectric pellets or beads filled within the reactor are not limited to BaTiO3, but include glass, ceramic, pyrex and the like. This method is also characterized in that a catalytic process is performed simultaneously with the non-thermal plasma process by adding a Pt-Pd-Rh catalyst which is a noble metal catalyst (See U.S. Pat. No.4,954,320). Besides, Yamamoto has disclosed a technique using a non-thermal plasma reactor and a power supplier similar to those of Nunez et al. and Birmingham et al., but having a difference in that BaTiO3 beads filled between electrodes are coated, at surfaces thereof, with Pt, Pd, Rh, Co, Ni or V as transition metal group catalysts to reduce by-products produced in the non-thermal plasma process (See U.S. Pat. No.5,843,288).
As described above, Pt, Pd, Rh, Co, Ni and V belonging to the transition metal group are proposed as the catalysts used in the conventional non-thermal plasma processes. In this case, however, there are some problems in that (1) using these transition metal group catalysts in the high-electric field plasma reactor increases a possibility of generating an arc as an obstacle to the formation of stable non-thermal plasma, and (2) the noble metal catalysts used mainly among the transition metal group catalysts are expensive. Also, reducing the amount of coating of the transition metals causes a degradation in catalytic performance. This problem may be solved using an increased amount of dielectric pellets or beads. In this case, however, the non-thermal plasma techniques may not be applied to most industrial processes involving a larger flow rate of exhaust gas because a pressure drop within the reactor is greater. Thus, there is a desire to develop a catalyst that can be activated vigorously by non-thermal plasma while being electrically non-conductive or less conductive.
Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art, and it is an objective of the present invention to provide a method for treating a toxic compounds-containing gas by a non-thermal plasma, which removes a possibility of the generation of arc by providing catalysts being electrically less conductive, thereby realizing the generation of a stable non-thermal plasma at a low cost and an enhanced efficiency of the treatment.
To achieve this objective, there is provided a method for treating a toxic compounds-containing gas by a non-thermal plasma, the method comprising the steps of:
filling alkali earth metal catalysts and dielectrics within a plasma reactor, the alkali earth metal catalysts being made by substituting alkali earth metal cations for cations of aluminum/silicon molecular sieves used as carriers;
introducing a toxic compounds-containing gas into the plasma reactor; and
supplying power to the plasma reactor to generate a non-thermal plasma and thus decomposing or oxidizing the toxic compounds.